It is well known since before the days of motion pictures that one can create the illusion of a moving image by the display of a rapid succession of still images. This technique is used not only in motion pitures but also in television. Motion pictures and television images are generated by a "hard" switch between frames of the moving image from one still image to the next. This illusion of motion created by a rapid sequence of still images is also used in the computer graphic arts whereby the computer generates a plurality of digital still images which are stored in memory to be later converted into a sequence of video frames.
Occasionally the image of one scene of a television program may be dissolved into that from another scene by fading out the first scene and fading in the second scene. However, this dissolve typically takes place over several frame intervals with the images in each scene continuning to change during the dissolve.
A number of problems exist with the creation of an illusion of motion from a sequence of computer generated still images. Typically the graphics images has a relatively coarse resolution because the picture elements (pixels) are relatively large in size as compared to pixels of a conventional NTSC video image. Although high resolution system with very small, imperceptible pixels are available, their expense makes them impractical for many applications. The pixels have fixed locations and as an object moves either horizontally or vertically from one still image to the next, the object must move in a minimum increment of a single pixel. Depending upon the size of the pixels and the speed at which the still images change, the movement may be perceived as being jerky with a flicker in the simulated motion. The motion will appear more jerky as the number of separate still images displayed per second drops, and with larger pixel size display systems. For example, motion pictures are conventionally shot at 24 frames per second. However, the shutter on the projector operates at 48 frames per second displaying each frame twice to reduce the frame to frame flicker (or motion aliasing).
Two obvious approaches to solving the problem of jerkiness in the moving image would be to (1) decrease the pixel size or (2) increase the rate at which the still images are displayed. Decreasing the size of the pixel increases the number of pixels in the image necessitating a dramatic increase in the amount of memory required to store each still image and the amount of computing power necessary to generate each image. Very small pixel sizes are often impractical from an economic standpoint. If one is to increase the image display rate, a larger number of still images will have to be displayed within a given period of time thereby also necessitating increased memory size and computing capabilities. In some instances, especially those involving computer games, the human operator is given control over the speed of the motion. In this case the number of frames per second cannot be predetermined and the only obvious solution is to decrease the pixel size.
This jerkiness problem can also be addressed by the software that generates the sequence of images. However, this approach costs computing time which could be better spent in creating more complex or natural appearing images.